严重问题!打开网站，报CS0006:错误，望高手解答

不知为何，今天打开网站突然就报CS0006: 未能找到元数据文件“C:\WINDOWS\assembly\GAC_32\System.Web\2.0.0.0__b03f5f7f11d50a3a\System.Web.dll”的错误,昨天还没事的,由于情况紧急，望高手解答,谢谢!
注：服务器系统是Windows Server2003
程序是用VS2008 + Framework3.5开发的

编译器输出:

c:\windows\system32\inetsrv> "C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\csc.exe" /t:library /utf8output /R:"C:\WINDOWS\assembly\GAC_MSIL\CrystalDecisions.CrystalReports.Engine\10.2.3600.0__692fbea5521e1304\CrystalDecisions.CrystalReports.Engine.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\System.Configuration\2.0.0.0__b03f5f7f11d50a3a\System.Configuration.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\System.Design\2.0.0.0__b03f5f7f11d50a3a\System.Design.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\System.Drawing\2.0.0.0__b03f5f7f11d50a3a\System.Drawing.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\CrystalDecisions.ReportAppServer.ClientDoc\10.2.3600.0__692fbea5521e1304\CrystalDecisions.ReportAppServer.ClientDoc.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\System.ServiceModel.Web\3.5.0.0__31bf3856ad364e35\System.ServiceModel.Web.dll" /R:"C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\81489006\7d379016\assembly\dl3\6b66c8c6\ef579b89_f4adc901\EntityLayer.DLL" /R:"C:\WINDOWS\assembly\GAC_MSIL\System.Windows.Forms\2.0.0.0__b77a5c561934e089\System.Windows.Forms.dll" /R:"C:\WINDOWS\assembly\GAC_32\System.EnterpriseServices\2.0.0.0__b03f5f7f11d50a3a\System.EnterpriseServices.dll" /R:"C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\mscorlib.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\System.Web.Services\2.0.0.0__b03f5f7f11d50a3a\System.Web.Services.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\System.Web.Extensions.Design\3.5.0.0__31bf3856ad364e35\System.Web.Extensions.Design.dll" /R:"C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\81489006\7d379016\assembly\dl3\69bd9a8a\2b42062e_f6adc901\DataLayer.DLL" /R:"C:\WINDOWS\assembly\GAC_32\System.Web\2.0.0.0__b03f5f7f11d50a3a\System.Web.dll" /R:"C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\81489006\7d379016\assembly\dl3\5de44b52\00482ce8_de49c401\ewebsoft.DLL" /R:"C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\81489006\7d379016\assembly\dl3\2c34c9f5\b49582bc_becec901\BusinessLayer.DLL" /R:"C:\WINDOWS\assembly\GAC_MSIL\System\2.0.0.0__b77a5c561934e089\System.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\System.ServiceModel\3.0.0.0__b77a5c561934e089\System.ServiceModel.dll" /R:"C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\81489006\7d379016\assembly\dl3\60369e81\00523faf_8ee4c701\AspNetPager.DLL" /R:"C:\WINDOWS\assembly\GAC\CrystalDecisions.Enterprise.InfoStore\10.2.3600.0__692fbea5521e1304\CrystalDecisions.Enterprise.InfoStore.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\CrystalDecisions.ReportSource\10.2.3600.0__692fbea5521e1304\CrystalDecisions.ReportSource.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\System.Web.Extensions\3.5.0.0__31bf3856ad364e35\System.Web.Extensions.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\System.Web.Mobile\2.0.0.0__b03f5f7f11d50a3a\System.Web.Mobile.dll" /R:"C:\WINDOWS\assembly\GAC_32\System.Data.OracleClient\2.0.0.0__b77a5c561934e089\System.Data.OracleClient.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\System.WorkflowServices\3.5.0.0__31bf3856ad364e35\System.WorkflowServices.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\CrystalDecisions.Web\10.2.3600.0__692fbea5521e1304\CrystalDecisions.Web.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\System.Runtime.Serialization\3.0.0.0__b77a5c561934e089\System.Runtime.Serialization.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\System.IdentityModel\3.0.0.0__b77a5c561934e089\System.IdentityModel.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\CrystalDecisions.Shared\10.2.3600.0__692fbea5521e1304\CrystalDecisions.Shared.dll" /R:"C:\WINDOWS\assembly\GAC_32\System.Data\2.0.0.0__b77a5c561934e089\System.Data.dll" /R:"C:\WINDOWS\assembly\GAC\CrystalDecisions.Enterprise.Desktop.Report\10.2.3600.0__692fbea5521e1304\CrystalDecisions.Enterprise.Desktop.Report.dll" /R:"C:\WINDOWS\assembly\GAC_MSIL\System.Xml\2.0.0.0__b77a5c561934e089\System.Xml.dll" /R:"C:\WINDOWS\assembly\GAC\CrystalDecisions.Enterprise.Framework\10.2.3600.0__692fbea5521e1304\CrystalDecisions.Enterprise.Framework.dll" /out:"C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\81489006\7d379016\App_Code.znwrlcs_.dll" /D:DEBUG /debug+ /optimize- /w:4 /nowarn:1659;1699;1701 "C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\81489006\7d379016\App_Code.znwrlcs_.0.cs" "C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\81489006\7d379016\App_Code.znwrlcs_.1.cs"


Microsoft (R) Visual C# 2005 编译器 版本 8.00.50727.1433
用于 Microsoft (R) Windows (R) 2005 Framework 版本 2.0.50727
版权所有 (C) Microsoft Corporation 2001-2005。保留所有权利。

error CS0006: 未能找到元数据文件“C:\WINDOWS\assembly\GAC_32\System.Web\2.0.0.0__b03f5f7f11d50a3a\System.Web.dll”